Although a power cable, which uses, as an insulation layer, a polymer insulator such as, for example, cross-linked polyethylene (XLPE), is used, due to the problem in which a space charge is created in a direct-current high electric field, a paper-insulated cable in which an insulation layer is formed by impregnating an insulating paper, which is wound to surround, for example, a conductor, with an insulation oil, is used as an extra-high voltage direct-current power transmission cable.
Examples of the paper-insulated cable may include an oil filled (OF) cable, which uses circulation of a low-viscosity insulation oil, and a mass impregnated non draining (MIND) cable, which is impregnated with a high-viscosity insulation oil. The OF cable has a limitation on the length along which a hydraulic pressure is transferred for the circulation of the insulation oil, and thus is not suitable for use as a long-distance power transmission cable. In particular, the OF cable is also not suitable for a submarine cable because it is difficult to install an insulation oil circulation facility underwater.
Therefore, the MIND cable is commonly used as a long-distance direct-current power transmission or submarine extra-high voltage cable.
Such a MIND cable is formed by surrounding an insulating paper in multiple layers when forming an insulation layer. As the insulating paper, for example, Kraft paper may be used, or a semi-synthetic paper may be used, in which Kraft paper and a thermoplastic resin such as, for example, a polypropylene resin, are stacked one above another.
In the case of a cable in which only Kraft paper is wound and is impregnated with an insulation oil, when the cable is used (upon electrical conduction), variation in temperature occurs due to the current that flows through a cable conductor, from an insulation layer portion on the radially inner side, i.e. on the inner semiconductive layer side to an insulation layer portion on the radially outer side, i.e. on the outer semiconductive layer side. Accordingly, the viscosity of the insulation oil in the insulation layer portion on the inner semiconductive layer side, which has a relatively high temperature, is reduced, whereby the insulation oil undergoes thermal expansion and moves to the insulation layer portion on the outer semiconductive layer side. Then, when the temperature drops, the viscosity of the insulation oil, which has moved due to thermal expansion, is increased, and thus the insulation oil has difficulty in moving to the original position thereof, whereby a de-oiling void may be formed in the insulation layer portion on the radially inner side, i.e. on the inner semiconductive layer side. Since the de-oiling void may cause an electric field to be concentrated thereon due to the absence of the insulation oil, for example, partial discharge or insulation destruction may occur in the vicinity of the void, which may reduce the lifespan of the cable.
However, when the insulation layer is formed of the semi-synthetic paper, the thermoplastic resin such as, for example, a polypropylene resin, which is not impregnated with the insulation oil when the cable is used, undergoes thermal expansion, which may suppress the flow of the insulation oil. In addition, since the polypropylene resin has an insulation resistance greater than that of the Kraft paper, even if the de-oiling void is formed, a voltage applied thereto may be attenuated.
In addition, the polypropylene resin, which is not impregnated with the insulation oil, may prevent the insulation oil from moving in the cable diametric direction due to the weight thereof, and may further suppress the movement of the insulation oil since the polypropylene resin undergoes thermal expansion according to the impregnation temperature upon the manufacture of the cable or the operating temperature when the cable is used, thereby applying surface pressure to the Kraft paper.
Meanwhile, for example, in Japanese Patent Laid-Open Publications No. 2010-097778, No. 2013-098136 and No. 2011-216292, a semi-synthetic paper and Kraft paper are mixed with each other in order to suppress the formation of a de-oiling void described above and to prevent an electric field from being concentrated immediately above a conductor and immediately below a sheath. However, in this case, optimum insulation design, i.e. realization of target resistance of an insulation layer and the minimum thickness of the insulation layer may be difficult, which results in an increase in the thickness of the insulation layer or a reduction in the lifespan of the cable due to the reduced dielectric strength. In addition, since a resin that constitutes the semi-synthetic paper, such as a polypropylene resin, is vulnerable to heat, upon a cable connection process, more particularly, upon a lead pipe connection process, the insulation layer may be degraded by the heat generated during welding, which may further reduce the lifespan of the cable.
Therefore, there is an urgent demand for a power cable in which an insulation layer itself may have a high dielectric strength, an electric field to be applied to the insulation layer may be effectively buffered, degradation of the insulation layer may be prevented when the cable is used or during a connection process, resulting in an increase in the lifespan of the power cable, and the thickness of the insulation layer may be minimized, resulting in a reduction in the outer diameter of the power cable, whereby the power cable may achieve improvements in, for example, flexibility, ease of installation, and workability.